Jose Mourinho has named a very young squad; goalscorer Mason Greenwood, 16, midfielder James Garner, 17, winger Tahith Chong, 18, plus defender RoShaun Williams, midfielder Ethan Hamilton and forward Joshua Bohui, all 19, have all travelled plus 17-year-old playmaker Angel Gomes, who has already made two appearances.
He will become Ajax's record signing and returns to the club a year after playing against them in United's Europa League final win in Sweden.
It represents a profit for United on a player who joined from Ajax for £13.8m in 2014.
Sources have told ESPN FC the deal will be worth £14.1 million, which could rise to £18.5m with add-ons.
GettyDaley Blind leaves United after four seasons at Old Trafford
Do you think United made the right decision to get rid of Blind?
Blind, a Dutch global, previously spent four years with the Ajax senior club before moving over to United four summers ago. (AFP/Getty Images) Blind was in pre-season training last week. With Ajax he clinched four Dutch league titles.
United's World Cup stars are now on holiday so the majority of the pre-season squad were youngsters meaning there's no real other reason for Blind to miss out.
"I'm really excited for this new challenge and my target is to meet the demands of the Premier League", the 26-year-old said on West Ham's official website.